Wash the fruit. Peel the grapefruit with a vegetable peeler and cut the skin into fine shreds. Cut all the fruit in half and juice it. Put the juice in a large bowl. Add enough water to bring up the volume to 2.5 litres (4½ pints). Scoop the flesh out of the grapefruit and tie it in a muslin bag. Add the bag to the bowl. Shred the orange skins as finely as possible and put them into the bowl along with the grapefruit rind. Leave overnight to soak.

Cook this mixture the next day until the shredded skin is completely soft. It will take about an hour and a half. Strain the jam through a sieve (keeping the shreds) and measure the liquid, making sure you squeeze all the pectin-filled juice from the bag. You should have about 1.5 litres (2¾ pints). Top up with water or reduce if not. Discard the bag.

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To sterilise your jars wash them and the lids in warm soapy water and place in an oven preheated to 110°C/225°F/gas mark ¼ for half an hour. It's easier to lift them in and out if you put them in a large roasting-tin. Leave them in the oven while you finish the jam. The jam has to be potted in warm sterilised jars. You should also sterilise the ladle you use and any jam funnel in boiling water.

Add the lemon juice and sugar to the pan and heat gently to dissolve. Bring to the boil and skim off any scum that appears. Add the shreds from the sieve and bring back to the boil. Check for setting point on a sugar thermometer – it sets at 104.5°C. When you have reached this temperature, perform the wrinkle test: put a teaspoonful of the mixture on the cold plate, leave it to cool for a minute then see if it wrinkles when pushed with your finger. Take the pan off the heat while you do the test, so you don't overcook the jam. If it hasn't reached setting point, put it back on the heat for four minutes and try again.

Check for flavour, adding a bit more lemon juice to lift the marmalade if you think it needs it. Stir and remove any scum. Leave to cool for about 12 minutes then pot in warm, sterilised jars (see the lime recipe) and seal.